Presently, about 95% of the 9 million tons of hydrogen produced in the United States uses a thermal process with natural gas as the feedstock. The most common process involves steam methane reforming (SMR) and water-gas shift reactions (WGS) at high temperature. Hydrogen gas must then be separated from the resultant mixed gas stream.
SMR is an endothermic process where methane and water are converted into hydrogen and carbon monoxide by the equation:CH4+H2O⇄3H2+CO.
WGS is an exothermic process that converts carbon monoxide and water to hydrogen and carbon dioxide:CO+H2O⇄H2+CO2.
Coupling the SMR to the WGS reaction yields a net reaction of:CH4+2H2O⇄4H2+CO2.
In addition to or in place of the methane, higher hydrocarbons or alcohols can be used to generate H2 by SMR and WGS, as illustrated for alkanes and mono hydroxy substituted alcohols:CnH2n+2+2nH2O⇄3n+1H2+nCO2 CnH2n+1OH+2n−1H2O⇄3nH2+nCO2.
Membrane reactor technology allows economic production of high purity hydrogen from natural gas, gasified coal, biomass, and other hydrocarbon feedstocks by coupling steam reforming and hydrogen transport in one step. Removal of product hydrogen continuously through the membrane shifts the equilibrium toward increased hydrogen production. Palladium metal alloy membranes have been available for hydrogen production for several decades, but these membranes are expensive and require large areas for adequate fluxes in commercial applications. In addition to industrial hydrogen production, efforts have been made to use SMR and WGS reactors and hydrocarbon fuels with hydrogen fuel cells in automotive applications to exploit the efficiencies of hydrogen fuel cell, which are generally more than twice that of internal combustion engines.
SMR is typically run at steam concentrations higher than the reaction stoichiometry to improve conversion. When the molar water-to-carbon ratio is large, WGS takes place in the same reactor allowing conversion of the hydrocarbons to H2 and CO2. Additionally, reactors that promote SMR and WGS must be robust as a number of secondary reactions, such as carbon formation, can occur that are detrimental to the performance of the reactor. The use of higher stoichiometry of water-to-carbon can significantly diminish the formation of undesired byproducts of these reactions. Higher temperatures also diminish the observance of these byproducts. As the SMR reaction is endothermic, high temperatures are favorable for the promotion of the reaction. On the other hand, WGS is mildly exothermic and reversible and the equilibrium constant for the formation of hydrogen is greater at lower temperatures, where the reaction rate is low. Hence, reaction is best carried out at higher temperatures in a manner that hydrogen is readily and rapidly removed from the reactor by having a high flux rate through the membrane to drive the equilibrium reaction to high conversion.
Perovskite-type oxides (e.g. BaCe1-xMxO3, where M is a metal dopant) have been shown to have high proton conductivities at elevated temperature, with protonic conductivities on the order of 10−2O−1 cm−1 at 600° C. As the hydrogen permeates through the membrane as a proton, separation selectivity for hydrogen is nearly absolute, allowing the collection of extremely pure hydrogen. The potential permeation flux rate of these materials is also extremely high if sufficient electronic conductivity can be achieved.
BaCe1-x-MxO3-type protonic conductors have insufficient electronic conductivity to balance the transport of charge through the material. To address these problem similar type protonic conductors, particularly BaCe1-xGdxO3, have been used to form a two phase proton and electric conductor where Pd acts as the electron conductor phase, as disclosed in Wachsman et al., U.S. Pat. No. 6,235,417. A subsequent patent to Wachsman et al., U.S. Pat. No. 6,296,687 discloses a mixed protonic-electronic conducting material useful as a H2 permeable membrane or electrode that comprises BaCe1-xMxO3-type conductors when M is a multivalent dopant metal.
Unfortunately, these systems have not demonstrated sufficient chemical stability for many commercial applications involving hydrogen production. There remains the need for improved stable membranes where high flux is achieved. Additionally, it is desirable to use such membranes in a reactor where a single high temperature is employed for hydrogen production and separation. It is also desirable to readily fabricate such a ceramic membrane reactor.